Hydraulically operated clutches are used for shifting gears in modern automatic transmissions of automobiles. To enable these switching operations to take place smoothly and without being noticeable to the driver, it is necessary to set the hydraulic pressure at the clutches with maximum precision according to predefined pressure ramps. Electromagnetically operated pressure control valves are used for this purpose. These valves may be designed either as seat valves or slide valves. As a rule, both structural designs have three hydraulic ports for inflow, control pressure and return flow.
In slide valves, the use of damping diaphragms or damping throttles has proven to be particularly advantageous for showing the necessary damping action. This takes advantage of the effect that hydraulic oil is displaced during movement of the control piston. When this hydraulic oil flows through a damping diaphragm or damping throttle, a pressure drop sets in which damps the movement of the control piston as a function of speed. A diaphragm or throttle having a shorter length compared to the hydraulically effective cross section is particularly advantageous because the damping effect is virtually independent of the oil temperature or the viscosity of the hydraulic oil. A sufficiently constant damping behavior over a wide range of operating temperatures is obtained thereby.
Numerous examples of conventional slide valves use damping diaphragms and/or damping throttles of this type.
A slide valve having an internal damping diaphragm and an external damping diaphragm in the working port of the pressure control valve is described in German Patent No. DE 198 29 549 B4.
In the slide valve described in U.S. Patent Application Publication No. 2009/0000677 A1, the damping diaphragm is situated in the return port.
A slide valve having a damping diaphragm at the tank port and a damping throttle in the working port is described in U.S. Patent Application Publication No. 2008/0072978 A1.
A pressure control valve is described in German Patent No. DE 103 25 070 A1 which has two damping ends in the working pressure port adjacent to the end face of the control piston.
The disadvantage of the conventional approaches in which the damping diaphragm is situated in the working port, is the fact that this necessitates additional sealing points on the pressure control valve. If the working pressure is namely supplied not only to the direct consumer port but also to the damping diaphragms, a corresponding seal must also be provided in that location, or additional leakage quantities must be taken into account.
In addition, a force effect of the pressure upon the piston is present whenever the damping diaphragms are used in the area of the pressure control valve to which pressure is applied. This force effect is undesirable and may be reduced by not using the entire piston surface of the control piston as the displacement volume but rather only an annular surface in the piston. However, this also reduces the displaced volume and thus the damping effect.
If diaphragm damping is implemented in the unpressurized tank port, the problem arises that filling this unpressurized chamber with oil is not reliably ensured.